Fast-neutron nuclear reactors cooled by a liquid metal, such as sodium, comprise a core consisting of assemblies of elongate prism-shaped form immersed in liquid sodium contained in the reactor vessel.
It may be necessary to extract fuel assemblies from the reactor core within the vessel, for example in order to replace spent or defective assemblies by new assemblies. The assemblies which are taken out of the reactor vessel are in the irradiated state and generate radioactive radiation and an emission of heat.
The fuel assemblies taken out of the vessel of the nuclear reactor must therefore be arranged in containers affording biological protection of the environment in which the assemblies are displaced or stored temporarily.
In carrying out the transfer of the fuel assemblies of fast-neutron nuclear reactors, it is customary to use handling flasks comprising a solid body which is made of a material absorbing nuclear radiation and in which is formed a channel passing through the body of the flask in its longitudinal direction and in a central position and serving as a receptacle for a fuel assembly introduced into the handling flask.
The channel opens out via a sealingly closable orifice at one of the ends of the body of the flask to make it possible to introduce an assembly into the flask and to extract it. During operations of introducing a fuel assembly into the flask and extracting it therefrom, the flask is arranged in such a way that the channel receiving the fuel assembly is in vertical position. The operations of introducing and extracting a fuel assembly are executed by means of a lifting set comprising winches associated with the flask and making it possible for the assembly fastened to a grab at its upper end to be displaced within the channel passing through the flask and in its axial extension.
The well shut-off device arranged in the lower part of the flask is generally connected to a corresponding device associated, for example, with a fuel-assembly passage well passing through the closing slab of the vessel of the nuclear reactor.
It is obvious that the operations of transferring the fuel assemblies between the reactor vessel and a handling flask must be conducted with a very high degree of safety, since incidents, such as the fall of a fuel assembly during its transfer, a failure of the lifting means of the assembly during transfer or a lack of cooling of the assembly, can cause damage to the fuel assembly or to some components of the reactor, a considerable delay in the handling operations or other harmful consequences resulting in an increase in the operating costs of the nuclear reactor.
It may also be necessary to employ emergency procedures which involve an extremely high outlay and which can require the intervention of operators in zones subjected to irradiation.
The lifting means associated with a fuel-assembly handling flask are therefore designed so as to include some redundancy in terms of their capabilities of holding and displacing the load consisting of the fuel assembly fastened to the grab being displaced in the channel of the flask.
Such lifting devices generally comprise a double winch installed in the upper part of the flask and consisting of two motor shafts carrying a cable-winding drum at each of their ends.
Two cables are each wound on two drums driven by different motor shafts and ensure the suspension and displacement of a grab comprising load attachment means. The grab has a support, on which are mounted pulleys which interact with the cables in order to ensure the suspension and displacement of the load.
This device, which affords great operating safety in that it has redundancy making it possible to limit the consequences of a cable break or of a motor failure, has the disadvantage of considerably increasing the bulk of the flask, the upper part of which carries the set of lifting means. The mass of these lifting means therefore means that the center of gravity of the flask as a whole is located at an appreciable height, this having an effect on the anti-seismic devices to be provided.
To make it easier to handle the flask during the transfer of a fuel assembly and to store flask during the periods when it is not being used, it is entirely desirable to reduce the volume of the flask as much as possible and to lower the position of the center of gravity.